1. Field of the Invention
The present invention relates to a printing apparatus which prints an image based on image data on a printing medium by discharging ink droplets from ink orifices formed in a printhead, and a control method therefor. More particularly, the present invention relates to a printing apparatus capable of obtaining a high-quality image by correcting dot displacement caused by the setting angle of the printhead or the like, and a control method therefor.
2. Description of the Related Art
An inkjet printing apparatus generally comprises a printhead in which ink orifices and printing elements such as heaters or piezoelectric elements serving as energy generation means for discharging ink droplets are arrayed in correspondence with each other. The inkjet printing apparatus prints an image on a printing medium by repeating print scanning of discharging ink droplets to a printing area while moving the printhead in the main scanning direction, and conveyance of the printing medium in the sub-scanning direction intersecting the main scanning direction.
Due to the rise of cost of the power supply and the like, it is difficult to equip the inkjet printing apparatus with a power capacity enough to simultaneously discharge ink droplets from all the ink orifices of each ink orifice array of the printhead. Thus, the printing elements are time-divisionally driven. The time-divisional driving will be explained. The printing elements of each ink orifice array are divided into a plurality of groups, and printing elements in each group are assigned to different blocks. Printing elements are sequentially driven for the respective blocks, and all the printing elements are driven by going around all the blocks. This time-divisional driving is repeated in print scanning in the main scanning direction, printing in a printing area corresponding to one scanning.
In the inkjet printing apparatus, the printhead may slantingly be mounted in the inkjet printing apparatus due to a mounting error when mounting the printhead in the inkjet printing apparatus or an error when assembling the printhead. In some cases, dot displacement corresponding to the slanting angle, i.e., so-called slanting displacement may occur.
The slanting displacement will be described in detail with reference to FIGS. 33 and 4.
FIG. 33 shows the arrangement of dots formed on a printing medium 12 when the printhead is ideally mounted in the inkjet printing apparatus without any slanting displacement. In FIG. 33, a printhead 11 is mounted in the inkjet printing apparatus with an ink orifice array arranged parallel to the sub-scanning direction indicated by an arrow B. The printhead 11 prints while moving on the printing medium 12 from left to right along the main scanning direction indicated by an arrow A. The printing medium 12 is conveyed in the direction of the arrow B. The upper side in FIG. 33 is the upstream side in the sub-scanning direction, and the lower side is the downstream side in the sub-scanning direction.
Printing elements corresponding to 128 ink orifices 13 of the printhead 11 are divided into 8 groups 0 (G0) to 7 (G7) each including 16 printing elements. Printing elements in each group are assigned to different blocks, and printing elements in the same blocks are sequentially driven. In FIG. 33, printing elements are divided into groups 0 to 7 by 16 printing elements from a printing element on the upstream side in the sub-scanning direction. Printing elements in each group are assigned to blocks 0 to 15 sequentially from a printing element on the upstream side in the sub-scanning direction. Printing elements are driven in the driving sequence of block 0→1→2→3→4→5→6→7→8→9→10→11→12→13→14→15 in one cycle.
If there is no slanting displacement, dots formed by 1-cycle driving of printing elements in blocks 0 to 15 fall within the area of the same column (width of one pixel). FIG. 33 shows the arrangement of dots formed on a printing medium when printing elements are driven in the order of blocks 0 to 15 and image data of three, first to third columns are assigned to the printing elements. Dots formed by 1-cycle driving of printing elements of each group are arranged in a predetermined area (same column), obtaining an image of a high printing quality.
FIG. 4 shows a dot arrangement when the printhead is mounted with a slope in the inkjet printing apparatus and slanting displacement occurs upon printing the same image as that in FIG. 33. FIG. 4 shows four columns of dots formed by printing elements in groups 4 to 7 in FIG. 4. In the following description, it is assumed that dots of only three left columns in FIG. 4 are formed by the printing elements of these groups. As shown in FIG. 4, dots formed on the upstream and downstream sides by printing elements assigned to the same block deviate from each other in the main scanning direction. Further, dots are formed at positions deviated from a column in which they should be originally arranged. For example, four dots corresponding to blocks 0 to 3 in group 2 are formed at positions deviated from a column area where they should be originally arranged. If slanting displacement occurs, dots are formed at positions deviated from an area where they should be originally arranged, lowering the image quality.
To prevent this, there is proposed a technique of correcting slanting displacement. More specifically, an inkjet printing apparatus comprises a means for detecting information on slanting displacement. The discharge timing of the printhead is changed based on the detected information on slanting displacement.
Japanese Patent Laid-Open No. 2004-09489 discloses a method of changing the discharge timing of the printhead by changing the position of image data to be read out from a printing buffer for each group in accordance with the slanting displacement in an inkjet printing apparatus which time-divisionally drives printing elements to discharge ink droplets.
The slanting displacement correction method described in Japanese Patent Laid-Open No. 2004-09489 will be explained with reference to FIGS. 34 and 4.
The inkjet printing apparatus has the same configuration as that shown in FIG. 33. Printing elements are divided into 8 groups 0 (G0) to 7 (G7) each including 16 printing elements. Printing elements in each group are assigned with block numbers of 0 to 15. Printing elements in each group are driven in the driving sequence of block 0→1→2→3→4→5→6→7→8→9→10→11→12→13→14→15. Also in this description, dots are formed based on image data of three, first to third columns by using all the ink orifices 13 of the printhead 11.
In this case, the printhead 11 is mounted and slanted clockwise with respect to a conveyance direction of a printing medium. Slanting displacement occurs so that the positions of dots formed by the ink orifices 13 at the two ends of the printhead 11 deviate from each other by one column in the main scanning direction. A method of correcting this slanting displacement will be explained.
A in FIG. 34 represents nozzle numbers NZL, selection blocks SBK, and image data (printing data) DATA assigned to printing elements of groups 0 (G0) to 7 (G7). B in FIG. 34 represents the arrangement of dots printed on a printing medium in correspondence with A in FIG. 34. The dot arrangement in B of FIG. 34 schematically shows the arrangement of dots formed on a printing medium when no slanting displacement occurs. The nozzle number is virtually assigned to each printing element, and nozzle numbers of 0 to 127 are assigned to printing elements sequentially from one on the upstream side in the sub-scanning direction.
In Japanese Patent Laid-Open No. 2004-09489, the read position of image data read out from the printing buffer changes for each group in accordance with the slanting displacement. As shown in FIG. 34, the read positions of image data assigned to printing elements of groups 4 to 7 change by one column in the main scanning direction.
More specifically, image data are assigned to printing elements of groups 0 to 3 so as to form dots in the areas of the first to third columns. To the contrary, image data are assigned to printing elements of groups 4 to 7 so as to form dots in the areas of the second to fourth columns by changing the image data read position.
FIG. 4 shows the arrangement of dots actually formed on a printing medium when the image data read position changes as shown in FIG. 34. FIG. 4 shows four columns of dots formed by printing elements of groups 4 (G4) to 7 (G7). Dots of three left columns are formed without changing the image data read position, and dots of three right columns are formed by changing the read position. That is, blank dots at the positions of groups 4 to 7 on a printing medium are formed when image data of the first column are assigned to printing elements of groups 4 to 7 without correction. By slanting displacement correction disclosed in Japanese Patent Laid-Open No. 2004-09489, dots of groups 4 to 7 are formed at positions offset from the positions of blank dots to the right by one column in the main scanning direction. This slanting displacement correction can reduce the deviation amount of dots formed by printing elements of the same block in the main scanning direction.
However, the correction method proposed in Japanese Patent Laid-Open No. 2004-09489 is to change the image data read positions of all printing elements of each group by one column. In a case where dots formed by printing elements of the same group include dots arranged in a column in which they should be originally arranged, and those not arranged in it, dots arranged in the column unless they are corrected are arranged at positions deviated from the column upon correction. Even if there are dots arranged at positions deviated from a column in which they should be originally arranged, the dots are not corrected as long as the number of such dots is small. Therefore, even a group including dots arranged at positions deviated from a column in which they should be originally arranged may not be corrected.
Attention is paid to the first column of 16 dots of group 5. Unless slanting displacement correction is performed, four dots corresponding to blocks 12 to 15 are arranged in the first column, and 12 dots corresponding to the remaining blocks 0 to 11 are arranged in an area on the left side of the first column. According to this slanting displacement correction, the image data read position is changed by one column for all the printing elements of the group by assigning image data of the first column at the timing when printing an image in the area of the second column. By this correction, four dots corresponding to blocks 12 to 15 are arranged at positions deviated from the first column in which these dots should be originally arranged, i.e., arranged in the area of the second column.
As summarized, the correction method proposed in Japanese Patent Laid-Open No. 2004-09489 can reduce the displacement amount of the dot arrangement in the main scanning direction. In some cases, however, this method cannot satisfactorily reduce slanting displacement, and dots arranged in an area where they should be originally arranged are arranged at positions deviated from the area.